[gdb/testsuite] Use require for ensure_gdb_index

Replace:
...
if { [ensure_gdb_index $binfile] == -1 } {
    return -1
}
...
with:
...
require {ensure_gdb_index $binfile} != -1
...
and consequently, add a missing UNTESTED message.

Tested on x86_64-linux, both with native and target board readnow.

gdb/testsuite/gdb.base/with-mf.exp

@@ -24,9 +24,7 @@ if {[prepare_for_testing "failed to prepare" $testfile "$srcfile $srcfile2" \
     return -1
 }
 
-if { [ensure_gdb_index $binfile] == -1 } {
-    return -1
-}
+require {ensure_gdb_index $binfile} != -1
 
 clean_restart $binfile
 

gdb/testsuite/gdb.dwarf2/gdb-add-index-symlink.exp

@@ -29,9 +29,7 @@ with_test_prefix non-symlink {
 	return -1
     }
 
-    if { [ensure_gdb_index $binfile] == -1 } {
-	return -1
-    }
+    require {ensure_gdb_index $binfile} != -1
 }
 
 # Regenerate exec without index.

gdb/testsuite/gdb.dwarf2/gdb-add-index.exp

@@ -27,9 +27,7 @@ if { [prepare_for_testing "failed to prepare" "${testfile}" \
     return -1
 }
 
-if { [ensure_gdb_index $binfile] == -1 } {
-    return -1
-}
+require {ensure_gdb_index $binfile} != -1
 
 # Ok, we have a copy of $binfile with an index.
 # Restart gdb and verify the index was used.

gdb/testsuite/gdb.dwarf2/imported-unit-runto-main.exp

@@ -74,9 +74,7 @@ if { [prepare_for_testing "failed to prepare" ${testfile} \
     return -1
 }
 
-if { [ensure_gdb_index $binfile] == -1 } {
-    return -1
-}
+require {ensure_gdb_index $binfile} != -1
 
 clean_restart ${binfile}
 

gdb/testsuite/gdb.rust/dwindex.exp

@@ -26,9 +26,7 @@ if {[prepare_for_testing "failed to prepare" $testfile $srcfile {debug rust}]} {
     return -1
 }
 
-if {[ensure_gdb_index $binfile "-dwarf-5"] == -1} {
-    return -1
-}
+require {ensure_gdb_index $binfile -dwarf-5} != -1
 
 gdb_exit
 gdb_start

gdb/testsuite/lib/gdb.exp

@@ -8247,6 +8247,10 @@ proc require { fn arg1 {arg2 ""} } {
 
     switch "$fn $op $val" {
 	"gdb_skip_xml_test == 0" { set msg "missing xml support" }
+	"ensure_gdb_index $binfile != -1" -
+	"ensure_gdb_index $binfile -dwarf-5 != -1" {
+	    set msg "Couldn't ensure index in binfile"
+	}
 	default { set msg "$fn != $val" }
     }